U.S. patent number 9,099,124 [Application Number 14/499,218] was granted by the patent office on 2015-08-04 for tunneling magnetoresistive (tmr) device with mgo tunneling barrier layer and nitrogen-containing layer for minimization of boron diffusion.
This patent grant is currently assigned to HGST Netherlands B.V.. The grantee listed for this patent is HGST Netherlands B.V.. Invention is credited to James Mac Freitag, Zheng Gao.
United States Patent |
9,099,124 |
Freitag , et al. |
August 4, 2015 |
Tunneling magnetoresistive (TMR) device with MgO tunneling barrier
layer and nitrogen-containing layer for minimization of boron
diffusion
Abstract
A tunneling magnetoresistance (TMR) device, like a magnetic
recording disk drive read head, has a nitrogen-containing layer
between the MgO barrier layer and the free and/or reference
ferromagnetic layers that contain boron. In one embodiment the free
ferromagnetic layer includes a boron-containing layer and a
trilayer nanolayer structure between the MgO barrier layer and the
boron-containing layer. The trilayer nanolayer structure includes a
thin Co, Fe or CoFe first nanolayer in contact with the MgO layer,
a thin FeN or CoFeN second nanolayer on the first nanolayer and a
thin Co, Fe or CoFe third nanolayer on the FeN or CoFeN nanolayer
between the FeN or CoFeN nanolayer and the boron-containing layer.
If the reference ferromagnetic layer also includes a
boron-containing layer then a similar trilayer nanolayer structure
may be located between the boron-containing layer and the MgO
barrier layer.
Inventors: |
Freitag; James Mac (Sunnyvale,
CA), Gao; Zheng (San Jose, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
HGST Netherlands B.V. |
Amsterdam |
N/A |
NL |
|
|
Assignee: |
HGST Netherlands B.V.
(Amsterdam, NL)
|
Family
ID: |
53719013 |
Appl.
No.: |
14/499,218 |
Filed: |
September 28, 2014 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01R
33/093 (20130101); G11B 5/39 (20130101); G01R
33/098 (20130101); G11B 5/3909 (20130101) |
Current International
Class: |
G11B
5/39 (20060101) |
Field of
Search: |
;360/324.1,324.11,324.12,324.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Evans; Jefferson
Attorney, Agent or Firm: Berthold; Thomas R.
Claims
What is claimed is:
1. A tunneling magnetoresistive (TMR) device comprising: a
substrate; a first ferromagnetic layer on the substrate; a
tunneling barrier layer consisting essentially of MgO on the first
ferromagnetic layer; and a second ferromagnetic layer on the
tunneling barrier layer; wherein one of said first and second
ferromagnetic layers is a free ferromagnetic multilayer having an
in-plane magnetization direction substantially free to rotate in
the presence of an external magnetic field, the free ferromagnetic
multilayer comprising a boron-containing layer and a
nitrogen-containing layer between the barrier layer and the
boron-containing layer; and wherein the other of said first and
second ferromagnetic layers is a reference ferromagnetic layer
having an in-plane magnetization direction substantially prevented
from rotation in the presence of an external magnetic field.
2. The device of claim 1 wherein said free ferromagnetic multilayer
further comprises a first nanolayer in contact with the barrier
layer and selected from Co, Fe and an alloy consisting of Co and
Fe, and wherein said nitrogen-containing layer is a second
nanolayer in contact with the first nanolayer and selected from a
FeN alloy and a CoFeN alloy.
3. The device of claim 2 wherein the first and second nanolayers
have a combined thickness greater than or equal to 2 .ANG. and less
than or equal to 8 .ANG..
4. The device of claim 2 wherein said FeN alloy has a composition
of the form Fe.sub.(100-x)N.sub.x, where x is between 1 and 50
atomic percent, and wherein said CoFeN alloy has a composition of
the form (Co.sub.xFe.sub.(100-x)).sub.(100-y)N.sub.y, where x is
between 20 and 80 atomic percent and y is between 1 and 50 atomic
percent.
5. The device of claim 2 wherein said free ferromagnetic multilayer
further comprises a third nanolayer in contact with the second
nanolayer and selected from Co, Fe and an alloy consisting of Co
and Fe.
6. The device of claim 1 wherein said boron-containing layer is an
alloy comprising Co, Fe and B.
7. The device of claim 6 wherein said boron-containing layer is an
alloy further comprising Ta.
8. The device of claim 1 wherein said free ferromagnetic multilayer
is the first ferromagnetic layer, and said reference ferromagnetic
layer is the second ferromagnetic layer.
9. The device of claim 1 wherein said reference ferromagnetic layer
is the first ferromagnetic layer, and said free ferromagnetic
multilayer is the second ferromagnetic layer.
10. The device of claim 1 wherein said reference ferromagnetic
layer is part of an antiparallel (AP) pinned structure comprising a
first AP-pinned (AP1) ferromagnetic layer having an in-plane
magnetization direction, a second AP-pinned (AP2) ferromagnetic
layer adjacent the tunneling barrier layer and having an in-plane
magnetization direction substantially antiparallel to the
magnetization direction of the AP1 layer, and an AP coupling (APC)
layer between and in contact with the AP1 and AP2 layers, wherein
the reference layer is the AP2 layer.
11. The device of claim 1 wherein said reference ferromagnetic
layer is a reference ferromagnetic multilayer comprising a
boron-containing layer and a nitrogen-containing layer between the
barrier layer and the boron-containing layer.
12. A tunneling magnetoresistive (TMR) read head comprising: a
first shield layer of magnetically permeable material; a reference
ferromagnetic layer on the first shield layer and having an
in-plane magnetization direction substantially prevented from
rotation in the presence of an external magnetic field; an
electrically insulating tunneling barrier layer consisting
essentially of MgO on and in contact with the reference layer; a
free ferromagnetic layer on the tunneling barrier layer and having
an in-plane magnetization direction oriented substantially
orthogonal to the magnetization direction of the reference layer in
the absence of an external magnetic field, the free ferromagnetic
layer comprising a first nanolayer in contact with the barrier
layer and selected from Co, Fe and an alloy consisting of Co and
Fe, a second nanolayer in contact with the first nanolayer and
selected from a FeN alloy and a CoFeN alloy, and a boron-containing
ferromagnetic layer on the second nanolayer; a capping layer on the
free ferromagnetic layer; and a second shield layer of magnetically
permeable material on the capping layer.
13. The read head of claim 12 wherein said FeN alloy has a
composition of the form Fe.sub.(100-x)N.sub.x, where x is between 1
and 50 atomic percent, and said CoFeN alloy has a composition of
the form (Co.sub.xFe.sub.(100-x)).sub.(100-y)N.sub.y, where x is
between 20 and 80 atomic percent and y is between 1 and 50 atomic
percent.
14. The read head of claim 12 further comprising a third nanolayer
in contact with the second nanolayer and selected from Co, Fe and
an alloy consisting of Co and Fe, and wherein said boron-containing
layer is in contact with the third nanolayer.
15. The read head of claim 14 wherein the combined thicknesses of
the first, second and third nanolayers is greater than or equal to
3 .ANG. and less than or equal to 12 .ANG..
16. The read head of claim 12 wherein said reference layer
comprises a boron-containing ferromagnetic layer, a first nanolayer
in contact with the barrier layer and selected from Co, Fe and an
alloy consisting of Co and Fe, and a second nanolayer between the
boron-containing layer and the first nanolayer and selected from a
FeN alloy and a CoFeN alloy.
17. The read head of claim 12 further comprising an antiparallel
(AP) pinned structure between the first shield layer and the
barrier layer and comprising a first AP-pinned (AP1) ferromagnetic
layer on the first shield layer and having an in-plane
magnetization direction, a second AP-pinned (AP2) ferromagnetic
layer having an in-plane magnetization direction substantially
antiparallel to the magnetization direction of the AP1, and an AP
coupling (APC) layer between and in contact with the AP1 and AP2
layers, and wherein said reference layer is said AP2 layer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to tunneling magnetoresistance
(TMR) devices, and more particularly to a TMR read head with a
magnesium oxide (MgO) tunneling barrier layer and a
boron-containing ferromagnetic layer.
2. Description of the Related Art
A tunneling magnetoresistance (TMR) device, also called a magnetic
tunneling junction (MTJ) device, is comprised of two ferromagnetic
layers separated by a thin insulating tunneling barrier layer. The
barrier layer is typically made of a metallic oxide that is so
sufficiently thin that quantum-mechanical tunneling of charge
carriers occurs between the two ferromagnetic layers. While various
metallic oxides, such as alumina (Al.sub.2O.sub.3) and titanium
oxide (TiO.sub.2), have been proposed as the tunneling barrier
material, the most promising material is crystalline magnesium
oxide (MgO). The quantum-mechanical tunneling process is electron
spin dependent, which means that an electrical resistance measured
when applying a sense current across the junction depends on the
spin-dependent electronic properties of the ferromagnetic and
barrier layers, and is a function of the relative orientation of
the magnetizations of the two ferromagnetic layers. The
magnetization of one of the ferromagnetic layers, called the
reference layer, is fixed or pinned, while the magnetization of the
other ferromagnetic layer, called the free layer, is free to rotate
in response to external magnetic fields. The relative orientation
of their magnetizations varies with the external magnetic field,
thus resulting in change in the electrical resistance. The TMR
device is usable as a memory cell in a nonvolatile magnetic random
access memory (MRAM) array and as TMR read head in a magnetic
recording disk drive.
FIG. 1 illustrates a cross-sectional view of a conventional TMR
read head 10. The TMR read head 10 includes a bottom "fixed" or
"pinned" reference ferromagnetic (FM) layer 18, an insulating
tunneling barrier layer 20, and a top "free" FM layer 32. The TMR
read head 10 has bottom and top nonmagnetic electrodes or leads 12,
14, respectively, with the bottom nonmagnetic electrode 12 being
formed on a suitable substrate. The FM layer 18 is called the
reference layer because its magnetization is prevented from
rotation in the presence of an applied magnetic field in the
desired range of interest for the TMR device, i.e., the magnetic
field from a recorded region of the magnetic layer in a magnetic
recording disk. The magnetization of the reference FM layer 18 can
be fixed or pinned by being formed of a high-coercivity film or by
being exchange-coupled to an antiferromagnetic (AF) "pinning"
layer. The reference FM layer 18 may be part of an antiparallel
(AP) pinned or flux-closure structure, where two ferromagnetic
layers are separated by an antiparallel coupling (APC) spacer layer
and thus antiparallel-coupled to form a flux closure, as described
in U.S. Pat. No. 5,465,185. The magnetization of the free FM layer
32 is free to rotate in the presence of the applied magnetic field
in the range of interest. In the absence of the applied magnetic
field, the magnetizations of the FM layers 18 and 32 are aligned
generally perpendicular in the TMR read head 10. The relative
orientation of the magnetizations of the FM layers 18, 32
determines the electrical resistance of the TMR device.
It is known that TMR devices with MgO tunneling barriers,
specifically Fe/MgO/Fe, CoFe/MgO/CoFe, and Co/MgO/Co tunnel
junctions, exhibit a very large magnetoresistance due to coherent
tunneling of the electrons of certain symmetry. However, MgO tunnel
junctions are required to have (001) epitaxy and perfect
crystallinity. The MgO barrier layer is typically formed by sputter
deposition and subsequent annealing, which forms the crystalline
structure. For CoFe/MgO/CoFe tunnel junctions it is known that
magnetoresistance is low due to inferior crystallinity of the MgO
barrier layer. However, it has been found that when boron (B) is
used in one or more of the reference and free ferromagnetic layers,
such as by the use of a thin amorphous CoFeB or CoFeBTa layer in a
multilayer structure, higher tunneling magnetoresistance
(.DELTA.R/R or TMR) is observed after annealing. The amorphous
CoFeB layer is known to promote high-quality crystallization of the
MgO into the (001) direction, and thus higher TMR.
Advanced TMR devices with even higher TMR will require a reduction
in the resistance-area product (RA), which means that the MgO
barrier layers will need to be made thinner. However, as the MgO
thickness decreases the TMR also decreases, which is believed due,
in part, to diffusion of boron into the MgO barrier layer. The
reduction in MgO thickness also results in an undesirable increase
in interlayer coupling field (H.sub.int), i.e., the strength of the
magnetic coupling field between the reference layer and the free
layer. A large H.sub.int degrades the performance of the TMR read
head. It is important to have low values of H.sub.int as the MgO
barrier layer thickness is reduced.
What is needed is a TMR device with a thin MgO barrier layer and
thus reduced RA, but with high TMR and low H.sub.int.
SUMMARY OF THE INVENTION
Embodiments of this invention relate to a TMR device with a thin
MgO barrier layer and a nitrogen-containing layer between the MgO
barrier layer and the free and/or reference boron-containing
ferromagnetic layers. In one embodiment the free ferromagnetic
layer includes a boron-containing layer and a trilayer nanolayer
structure between the MgO barrier layer and the boron-containing
layer. The trilayer nanolayer structure includes a thin Co, Fe or
CoFe first nanolayer in contact with the MgO layer, a thin FeN or
CoFeN second nanolayer on the first nanolayer and a thin Co, Fe or
CoFe third nanolayer on the FeN or CoFeN nanolayer between the FeN
or CoFeN nanolayer and the boron-containing layer. If the reference
ferromagnetic layer also includes a boron-containing layer then a
similar trilayer nanolayer structure may be located between the
boron-containing layer and the MgO barrier layer. TMR devices
according to embodiments of the invention exhibit greater values of
TMR and lower values of interlayer coupling field (H.sub.int) for
thin MgO barrier layers than TMR devices without the
nitrogen-containing layers.
For a fuller understanding of the nature and advantages of the
present invention, reference should be made to the following
detailed description taken together with the accompanying
figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view illustrating the structure of a
conventional tunneling magnetoresistive (TMR) read head.
FIG. 2 is a cross-sectional view illustrating the detailed
structure of a prior-art TMR read head.
FIG. 3 is a schematic illustrating a typical
reference-layer/MgO/free-layer structure with boron present in the
reference and free layers in a prior art TMR read head.
FIG. 4 is a schematic illustrating a reference-layer/MgO/free-layer
structure with boron present in the reference and free layers and
with a nitrogen-containing layer between the MgO barrier layer and
the boron-containing layer according to an embodiment of the
invention.
FIG. 5 is a graph of TMR as a function of resistance-area product
(RA) for test devices according to an embodiment of the invention
and for control devices.
FIG. 6 is a graph of interlayer coupling filed (H.sub.int) as a
function of RA for test devices according to an embodiment of the
invention and for control devices.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 2 is a cross-sectional highly schematic view illustrating the
structure of a prior-art TMR read head 100 like that used in a
magnetic recording disk drive. This cross-sectional view is a view
of what is commonly referred to as the air-bearing surface (ABS) of
the TMR read head 100. The TMR read head 100 includes a sensor
stack of layers formed between two ferromagnetic shield layers S1,
S2 that are typically made of electroplated NiFe alloy films. The
lower shield S1 is typically smoothened by chemical-mechanical
polishing (CMP) to provide a smooth surface for the growth of the
sensor stack. The sensor stack includes a ferromagnetic reference
layer 120 having a pinned magnetization 121 oriented transversely
(away from the page), a ferromagnetic free layer 110 having a
magnetization 111 that can rotate in the plane of layer 110 in
response to transverse external magnetic fields from a recording
disk, and an electrically insulating tunneling barrier layer 130,
typically magnesium oxide (MgO), between the ferromagnetic
reference layer 120 and ferromagnetic free layer 110.
The reference layer 120 may be a conventional "simple" or single
pinned layer that has its magnetization direction 121 pinned or
fixed, typically by being exchange coupled to an antiferromagnetic
layer. However, in the example of FIG. 2, the reference layer 120
is part of the well-known antiparallel (AP) pinned or flux-closure
structure, also called a "laminated" pinned layer, as described in
U.S. Pat. No. 5,465,185. The AP-pinned structure minimizes
magnetostatic coupling of the reference layer 120 with the free
layer 110. The AP-pinned structure includes the reference
ferromagnetic (AP2) layer 120 and a lower pinned ferromagnetic
(AP1) layer 122 that are antiferromagnetically coupled across an AP
coupling (APC) layer 123, such as Ru, Ir, Rh, or Cr, or alloys
thereof. Due to the antiparallel coupling across the APC layer 123,
the reference (AP2) and pinned (AP1) ferromagnetic layers 120, 122
have their respective magnetizations 121, 127 oriented antiparallel
to each other. As a result, the net magnetization of the AP2 and
AP1 ferromagnetic layers 120, 122 is so small that a demagnetizing
field induced by the flux closure structure in the ferromagnetic
free layer 110 is substantially minimized, and thus it becomes
feasible for the TMR read head to operate optimally.
Located between the lower shield layer S1 and the AP-pinned
structure are seed layer 125 and an antiferromagnetic (AF) pinning
layer 124. The seed layer 125 facilitates the AF pinning layer 124
to grow a microstructure with a strong crystalline texture and thus
develop strong antiferromagnetism. The seed layer 125 may be a
single layer or multiple layers of different materials. The AF
pinning layer 124 thus strongly exchange-couples to the
ferromagnetic pinned layer 122, and thereby rigidly pins the
magnetization 127 of the ferromagnetic pinned layer 122 in a
direction perpendicular to and away from the ABS. The antiparallel
coupling across the APC layer 123 then subsequently rigidly pins
the magnetization 121 of the ferromagnetic reference layer 120 in a
direction perpendicular to and towards the ABS, and antiparallel to
magnetization 127. As a result, the net magnetization of the
ferromagnetic AP2 and AP1 layers 120, 122 is rigidly pinned, and
thus the optimal operation of the TMR read head is ensured. Instead
of being pinned by an AF layer, the AP1 layer 122 may by itself be
a hard magnetic layer or have its magnetization 127 pinned by a
hard magnetic layer such as Co.sub.100-xPt.sub.x or
Co.sub.100-x-yPt.sub.xCr.sub.y (where x is between about and 8 and
30 atomic percent). The AP-pinned structure may also be
"self-pinned". In a "self pinned" sensor the AP1 and AP2 layer
magnetization directions 127, 121 are typically set generally
perpendicular to the disk surface by magnetostriction and the
residual stress that exists within the fabricated sensor.
Located between the ferromagnetic free layer 110 and the upper
shield layer S2 is a layer 112, sometimes called a capping or cap
layer. The layer 112 protects the ferromagnetic free layer 110 from
chemical and mechanical damages during processing, so that
ferromagnetic free layer 110 maintains good ferromagnetic
properties.
In the presence of external magnetic fields in the range of
interest, i.e., magnetic fields from written data on the recording
disk, while the net magnetization of the ferromagnetic layers 120,
122 remains rigidly pinned, the magnetization 111 of the
ferromagnetic free layer 110 will rotate in responses to the
magnetic fields. Thus when a sense current I.sub.S flows from the
upper shield layer S2 perpendicularly through the sensor stack to
the lower shield layer S1, the magnetization rotation of the
ferromagnetic free layer 111 will lead to the variation of the
angle between the magnetizations of the ferromagnetic reference
layer 120 and the ferromagnetic free layer 110, which is detectable
as the change in electrical resistance. Because the sense current
is directed perpendicularly through the stack of layers between the
two shields S1 and S2, the TMR read head 100 is a
current-perpendicular-to-the-plane (CPP) read head.
FIG. 2 also shows optional separate electrical leads 126, 113
between shields S1, S2, respectively, and the sensor stack. Leads
126, 113 may be formed of Ta, Ti, Ru, Rh or a multilayer thereof.
The leads are optional and may be used to adjust the
shield-to-shield spacing. If the leads 126 and 113 are not present,
the bottom and top shields S1 and S2 are used as electrical leads.
The ferromagnetic reference and free layers 120 and 110 are
typically formed of a CoFe, CoFeB or NiFe layer, or formed of
multiple layers comprising these films, while the ferromagnetic
pinned layer 122 is typically formed of CoFe alloys. The seed layer
125 is typically formed of multiple layers comprising
Ta/NiFeCr/NiFe, Ta/NiFe, Ta/Ru or Ta/Cu films. The AFM pinning
layer 124 is typically made of an FeMn, NiMn, PtMn, IrMn, PdMn,
PtPdMn or RhMn film. The cap layer 112 is typically made of Ru, Rh,
Ti, Ta or a multilayer thereof.
While the TMR read head 100 shown in FIG. 2 is a "bottom-pinned"
read head because the AP-pinned structure is below the free layer
110, the free layer 110 can be located below the AP-pinned
structure. In such an arrangement the layers of the AP-pinned
structure are reversed, with the AP2 layer 120 on top of and in
contact with the barrier layer 130.
MgO tunnel junctions are required to have (001) epitaxy and perfect
crystallinity. The MgO barrier layer is typically formed by sputter
deposition and subsequent annealing, which forms the crystalline
structure. It has been found that the use of a thin amorphous CoFeB
or CoFeBTa layer in one or both of the reference and free layer
results in higher tunneling magnetoresistance (.DELTA.R/R or TMR).
The as-deposited amorphous CoFeB layer is known to promote
high-quality crystallization of the MgO into the (001) direction,
and thus higher TMR after annealing. Thus FIG. 3 is a schematic
illustrating a typical reference-layer/MgO/free-layer structure
with boron present in the reference and free layers. Each of the
reference and free ferromagnetic layers is depicted as a thin
(e.g., between about 1-4 .ANG. thick) CoFe "nanolayer" adjacent the
MgO barrier layer, a CoFe layer and a CoFeB (and in some instances
CoHf, CoFeBTa, or other amorphous insertion layer) layer between
the nanolayer and the CoFe layer. The CoFeB layer has a typical
composition of (Co.sub.xFe.sub.(100-x)).sub.(100-y)B.sub.y, where
the subscripts represent atomic percent, x is between about 60 and
100, and y is between about 10 and 20. The total thickness of each
of the reference and free layers is typically between about 20 and
80 .ANG.. Other materials are well known for use in the reference
and free layers, such as Co or Fe nanolayers, NiFe alloys and
Heusler alloys.
Advanced TMR devices with even higher TMR will require a reduction
in the resistance-area product (RA), which means that the MgO
barrier layers will need to be made thinner. However, as the MgO
thickness decreases the TMR also decreases, which is believed, in
part, due to diffusion of boron into the MgO barrier layer. The
reduction in MgO thickness also results in an undesirable increase
in interlayer coupling field (H.sub.int), i.e., the strength of the
magnetic coupling field between the reference layer and the free
layer.
Embodiments of this invention relate to a TMR device with a thin
MgO barrier layer and a nitrogen-containing layer between the MgO
barrier layer and the free and/or reference boron-containing layers
that has been shown to reduce diffusion of the boron into the MgO
barrier layer. FIG. 4 is a schematic illustrating a
reference-layer/MgO/free-layer structure with boron present in the
reference and free layers according to an embodiment of the
invention. The MgO barrier layer has a typical thickness in the
range of about 6 to 10 .ANG.. Each of the reference and free layers
contains boron in the form of a CoFeB (or CoFeBTa) layer and a thin
(e.g., between about 1-4 .ANG. thick) nitrogen-containing nanolayer
between the MgO barrier layer and the boron-containing layer. In
the preferred embodiment the nitrogen-containing nanolayer is a
CoFeN nanolayer that is part of a trilayer nanolayer structure that
includes a thin (e.g., between about 1-4 .ANG. thick) CoFe
nanolayer in contact with the MgO layer and another thin (e.g.,
between about 1-4 .ANG. thick) CoFe nanolayer between the CoFeN
nanolayer and the CoFeB (or CoFeBTa) layer. The CoFe nanolayers
preferably have a composition of the form Co.sub.xFe.sub.(100-x)
where x is between about 20 and 80 atomic percent. In another
embodiment the CoFe nanolayers may be replaced by Co or Fe
nanolayers and the CoFeN nanolayers replaced by FeN nanolayers. The
combined thickness of the trilayer nanolayer structure is greater
than or equal to 3 .ANG. and less than or equal to 12 .ANG.. FIG. 4
depicts both the reference and free layers as containing boron;
however the TMR device according to embodiments of the invention
may have only the free layer or only the reference layer as
containing boron, in which case only that layer will have the
nitrogen-containing layer located between the MgO barrier layer and
the boron-containing layer. FIG. 4 also depicts the TMR device as a
"bottom-pinned" device because the reference layer is below the
free layer; however, the reference layer may be located above the
free layer, in the manner as described above in FIG. 2.
The TMR read head with the tunnel junction described above and
shown in FIG. 4 is fabricated in the conventional manner by
deposition of the layers in the sensor stack by sputter deposition
or other known thin-film disposition techniques. The CoFeN (or FeN)
nanolayers may be deposited by reactive sputtering from a CoFe (or
Fe) target, or co-sputtered from separate Co and Fe targets, in the
presence of nitrogen gas. The nitrogen concentration in the
reactive sputtering gas should preferably be between 5% and 50%,
which results in an estimated composition being of the form
(Co.sub.xFe.sub.(100-x)).sub.(100-y)N.sub.y (or
Fe.sub.(100-y)N.sub.y), where x is between about 20 and 80 atomic
percent and y is between about 1 and 50 atomic percent. Structural
analysis of the as-deposited films suggests a nitrogen-doped phase
with minor ordered phases appearing as the reactive nitrogen flow
increases. The structure is then annealed in the presence of an
applied magnetic field to set the direction of the magnetization of
the reference ferromagnetic layer. The annealing is typically done
at about 250 to 290.degree. C. for about 4 to 24 hours. The
annealing also forms the MgO barrier layer with the desired
crystallinity, but without significant diffusion of the boron into
the MgO barrier layer. In addition, some of the nitrogen in the
CoFeN layer can diffuse out of this layer into other surrounding
layers, including the barrier layer, during the annealing. After
deposition and annealing of the films, the stack is
lithographically patterned and etched to define the desired
dimensions for the read head.
TMR test devices according to an embodiment of the invention were
fabricated at the wafer level and compared with TMR control devices
for TMR (.DELTA.R/R) and H.sub.int. For both the test and control
devices the MgO barrier layer was formed on a Co nanolayer. The
primary portion of the free layer was a 15 .ANG. thick
(Co.sub.96Fe.sub.4)B.sub.20 layer. For the control devices the
structure between the MgO layer and the primary free layer portion
was a 5 .ANG. Co.sub.40Fe.sub.60 bilayer nanolayer structure
without nitrogen. For the test devices the structure between the
MgO layer and the primary free layer portion was a 2 .ANG.
Co.sub.50Fe.sub.50/2 .ANG. CoFeN/2 .ANG. Co.sub.50Fe.sub.50
trilayer nanolayer structure. Test and control devices were
fabricated with different values of RA.
FIG. 5 shows the measured values of TMR. As RA is decreased from
about 0.45 .OMEGA..mu.m.sup.2 to about 0.27 .OMEGA..mu.m.sup.2 the
TMR for the control devices decreases more rapidly than the TMR for
the test devices, and at low values of RA the test devices exhibit
higher TMR. This is believed due to a reduction in the migration of
the boron into the MgO layer and/or by replacement of boron atoms
in the MgO with nitrogen atoms during annealing.
FIG. 6 shows the measured values of H.sub.int. For all values of
RA, the test devices showed substantially lower absolute values of
H.sub.int than the control devices. In addition, the magnetic
properties of the test devices, such as coercivity and
magnetostriction, were not significantly different than the control
devices, thereby ensuring adequate stability in the read head.
While the present invention has been particularly shown and
described with reference to the preferred embodiments, it will be
understood by those skilled in the art that various changes in form
and detail may be made without departing from the spirit and scope
of the invention. Accordingly, the disclosed invention is to be
considered merely as illustrative and limited in scope only as
specified in the appended claims.
* * * * *